1. Field of the Invention
The present invention relates to a braking device, a controller, an operating force controller, and an optical device that can vary braking force.
2. Related Background Art
FIG. 10 is a diagram showing the principle of a first braking device of a first conventional art example. This braking device comprises a cylindrical member 1 that is externally driven and a belt-shaped member 27 that has one end fixed and that is so positioned that it is wound around the cylindrical member 1.
When the other end (free end) of the belt-shaped member 27 that is not fixed is pulled downward, as is indicated by an arrow in FIG. 10, the belt-shaped member 27 is pressed onto the external surface of the cylindrical member 1. The contact of the cylindrical member 1 with the belt-shaped member 27 generates force due to friction that acts as a braking torque.
FIG. 11 is a perspective view of a second braking device of a second conventional art example in which a braking torque is proportional to the number of revolutions and is variable. In this braking device, disks 29 that are mounted on a rotary shaft 28 are provided in a Newtonian fluid 2 that is held in a sealed container 33. As the rotary shaft 28 is rotated by an external device, shearing strain is induced in the Newtonian fluid 2 between semicircular disks 31, which are fitted around a control shaft 32 that is provided in parallel to the rotary shaft 28, and the disks 29, so that a braking torque that is proportional to the rotation of the rotary shaft 28 is generated.
Since, from the view of rotation, the value of the braking torque at a specific number of revolutions is proportional to an area where the disks 29 and the semicircular disks 31 overlap, the torque value can be adjusted by rotating the control shaft 32 and changing the overlap area.
As the braking device employs the resistance of the Newtonian fluid 2 that occurs due to shear strain force in proportion to the shear strain speed to generate the braking force, this braking force acts so that it is proportional to the number of revolutions. The torque-revolutions characteristic of the braking device is represented together with that of a general prime mover in FIG. 12. When the torque-revolutions characteristic in FIG. 12 is compared with the torque-revolutions characteristic of a prime mover in the first conventional art example in FIG. 13, it is apparent that the difference between inclinations in the graph for the prime mover and the braking device in the second conventional art example is substantially increased in comparison with those for the first conventional art example shown in FIG. 10.
Thus, with the above described arrangement, constant-speed operation of an entire system can be improved without requiring a speed detector and a feedback means. Further, since the operation speed corresponds to an intersection of lines in a graph for the braking torque-revolution characteristic of the braking device and the driving torque-revolution characteristic of the prime mover, the angle of rotation of the control shaft 32 is changed to alter the proportional coefficient for the braking torque-revolution characteristic of the braking device, thus providing an arbitrary driving speed. In other words, the braking device serves as a speed control means.
In the first conventional art example, however, the braking force generation source is derived from the friction that occurs between solid objects, i.e., a cylindrical member and a belt-shaped member, and the following problems occur.
(1) The first conventional art example is not appropriate for constant-speed rotation.
FIG. 13 is a graph showing the torque-revolutions characteristics for the first conventional braking device and a prime mover that is externally coupled with the braking device. As is shown in FIG. 13, since, in the braking device, the braking torque is caused by friction that occurs between the solid objects, a constant braking torque is generated, regardless of the number of revolutions.
On the other hand, a characteristic of a general prime mover, such as a motor, is that an internal loss causes a slight reduction in a generated torque as the number of revolutions is increased.
Consider a case where this braking device is coupled with a prime mover. Supposing that the braking device and the prime mover are rotated at revolutions V.sub.1, as is shown in FIG. 13, as the braking torque of the braking device is higher than the driving torque of the prime mover, the speed is decreased. When they are rotated at revolutions V.sub.2, however, the driving torque is larger than the braking torque.
Therefore, in a case where the braking device is linked to the prime mover, when the number of revolutions is V.sub.0, the driving torque of the prime mover and the braking torque of the braking device are balanced. At this time, the greater the inclinations for the characteristic of the braking device and for the characteristic of the prime mover, the more the torque, for which the specific number of revolutions is returned to the revolution number V.sub.0, is increased. The number of revolutions therefore recovers to the vicinity of the revolution number V.sub.0 even when there is a disturbance, and the first conventional art example is therefore appropriate for constant-speed rotation.
However, because in principle the braking torque of the first conventional braking device does not depend on the number of revolutions and the difference between the inclinations in graphs for the braking characteristic and the prime mover's characteristic can not be great, the recovery to constant-speed rotation is insufficient in most cases. Thus, in a machine for which constant-speed operation is required, an electric feedback is usually employed, instead of the braking device in the first conventional art, to change the torque-revolutions characteristic of the prime mover. To do this, however, expensive revolution detection means and feedback means must be provided. Further, as the magnitude of a feedback signal is small during low-speed operation, the feedback signal can not be precisely transmitted due either to the noise produced by an electric feedback means, or to the play in a mechanical feedback means, and speed constancy is unsatisfactory.
For an application, such as the zooming operation of a zoom lens, where it is required to maintain a constant speed as it is manipulated by a person, the improvement of the torque-revolution characteristic for the operating force exerted by an operator, who corresponds to a prime mover, is equivalent to the effort that is required of the operator. Therefore, in order to increase the operability level, no way is available other than the enhancement of the speed constancy that is derived from the improvement of the characteristic of the braking device, and a braking device is demanded for which the braking torque becomes greater in consonance with an increase in the number of revolutions.
Further, in the braking device in the first conventional art example that employs the force that is generated by friction between solid objects, stick-slip easily occurs during the low-speed operation and speed constancy is deteriorated.
(2) The braking device in the first conventional art example is not appropriate for a control section for which a high operability level and the perception of a preferable sensation by an operator are required.
For an operational section, such as a zooming section of a zoom lens for a television camera, for which a high operability level and the perception of a preferable sensation by an operator are required, a breaking device for which the operating force is variable has not yet been provided. When a sensory test was conducted with a control section, of which the operating force is fixed and for which the operability and the sensation that is experienced by an operator are excellent, and its torque-revolution characteristic was measured, the torque was increased in proportion to the number of revolutions. From this result, it is assumed that the increase of the operating force in proportion to operation speed is necessary in order to provide a high operability level and the perception of a desirable sensation by an operator. Because, as is described in (1) above, it is assumed that the speed constancy of a system, which consists of the control section that has this characteristic and the human efforts, greatly influences the operability level and the perception of an operational sensation. In the above conventional art, even though the operating force is variable, it is almost constant regardless of the operation speed, and its operability level and the sensation that is perceived by an operator are not preferable. This was confirmed by the results of the sensory test.
(3) Wear in members occurs.
Since the braking torque is generated by the rubbing together of solid objects, i.e., the cylindrical member and the belt-shaped member, both the shaft and the belt-shaped member wear out in time. Therefore, periodical maintenance and replacement of these items are required.
The second conventional art example has the following problems.
(4) A sealing mechanism is required.
As the second conventional art example is constructed by sealing Newtonian fluid in a container, sealing mechanisms 30 and 34 need to be located around the rotary shaft and the control shaft to prevent leakage of the Newtonian fluid. Since the shaft of the general sealing mechanism is pressed against by a flexible member, a large amount of friction occurs from the rubbing together of solid objects. Due to this friction, the braking torque in the second conventional art example, as well as in the first, does not depend on the number of revolutions. The feature whereby the braking torque of the braking device is proportional to the number of revolutions is lost, while the braking torque is little decreased from its constant value and the small torque value side of a braking torque control width is limited. This is equivalent to the braking torque at revolution 0 that is comparatively large, as is shown in FIG. 12.
(5) The structure in the second conventional art example is complicated.
The structure of the second conventional art example is more complicated than that of the first conventional art example. This results in an increase in manufacturing costs. Further, although the braking torque is reduced when bubbles occur in the Newtonian fluid, it is difficult to prevent the occurrence of bubbles during assembly because of the complicated structure.